Detect and report vacuous properties

[bts]

This is important for minimizing user confusion and surprise. Because
property assumes transaction success, when a function cannot
successfully run to completion, it is possible to "validate" any
property in this setting.

Consider some logical formula that corresponds to the Pact expression
(property some_property), where some_property can be anything:

tx_success => some_property

This is equivalent to:

¬tx_success Λ some_property

If tx_success is always false, then this expression is always true, no
matter the value of some_property.

We now detect such a condition where it is not possible for this
function to succeed (if called as a top-level function), and we produce
an error notifying the user that the property is moot:

Vacuous property encountered! There is no way for a transaction to succeed if it is calls this function from the top-level. Because all property expressions in Pact assume transaction success, in this case it would be possible to validate any property, even e.g. false.

Closes #623.

/cc @ggobugi27